1. Field of the Invention
The present invention relates to a focal point correction apparatus, and in particular relates to a focal point correction apparatus for use in cameras for example.
2. Description of the Prior Art
Conventionally, in automatic focusing mechanisms for cameras, focal point detecting devices have been constructed so as to detect a focusing condition of an optical image formed on a point that is optically equivalent to a predetermined fixed position to which a light sensitive surface of a film is to be disposed (referred to below as the "base focal position") and that is physically different from the light sensitive surface of the film, because it is not possible to directly detect the optical image formed on the film surface. On the other hand, the film is positioned within a camera by being pressed from the backside thereof with a pressure plate, for example such that the light sensitive surface of the film is accurately positioned at the base focal position.
However, it is difficult to properly position the light sensitive surface of the film at the base focal position due to the tendency of the film to curl along its lengthwise direction as a result of the film being wound and due to a curvature of the film across its breadth which is caused by changes in the surrounding environment such as changes in humidity, temperature, etc. Therefore, experience has shown that the pressure plate should be placed at a position where the image looks best on average.
For this reason, the condition of the film will have a direct effect on the extent of improper focusing, and in particular, this effect will be much stronger for the central portion of a film used in cameras using a relatively large size film. In other words, the extent of improper focusing is greater for such cameras using the relatively large size films.
For example, among the Brownie size films, there are the "120 film" which has paper backing and the "220 film" which has no paper backing, with these two films having different thicknesses relative to each other. As a result, cameras which are adapted to use these two films are provided with a switching mechanism making it possible to select two positions for the pressure plate, namely, one position suitable for the "120 film" and the other position suitable for the "220 film", so that both films may be properly positioned at the base focal position. Consequently, when photographing with this type of camera and a Brownie size film, it is necessary for the photographer to operate the switching mechanism of the camera in order to select the proper position for the particular type of film being used.
However, the above construction creates several problems for the photographer. Namely, there is the inconvenience of having to reset or check the switching mechanism each time the film is changed, and in the case where the photographer forgets to reset the switching mechanism when changing to a different type of film, improper focusing will result. Furthermore, even in the case where the switching mechanism is set at the proper setting for the thickness of the film being used, environmental factors, for example may give rise to curling or curvature of the film, and in this case, such problems cannot be dealt with in the same way as was mentioned previously above.